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12v power supply wiring grouping

Practical Harness Construction - Club Level

Discussion and questions related to the course Practical Harness Construction - Club Level


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When doing a total engine and chassis rewire what ways is best to group up relays? I find myself using quite a few relays when perhaps less would be fine. I run a seperate relay for the ecu only to start with so it has a dedicated supply, then quite a few relays that come on for enable power such as one for injectors, one for coils, one for boost control and idle speed, one for o2 heater, one for flexfuel sensor, could i share these enable power circuits over just maybe 2 relays providing i dont go over the relays amp rating?

Also is it best to put a fuse before or after a relay? Alot of literture i have read suggests before however with the fuse after the relay i find it better as i can use a single relay to supply a few differnt fuses and can protect each circuit seperatly.

It sounds like you're probably using more relays than strictly necessary. This isn't a bad thing but obviously adds to your costs and the time required. Everyone has their own preference around how to group the power feeds and there isn't strictly a right and wrong way to go about this. As a guide, you can consider the expected current draw of the circuits and then group them so that you're within the maximum current rating of the relay (usually 30 amps). I generally run the ECU power and other 12V feeds such as lambda sensor heater, boost solenoid, ethanol sensor etc off a single relay, then run fuel and ignition off a separate relay. Some people prefer to run the ignition 12v off the fuel pump relay so that the coils are only powered when the fuel pump is running.

As far as fusing goes, I personally prefer to fuse after the relay so that I can fuse individual circuits.

Thanks Andre. I have took your advise on board and constructed a little relay/fuse/switch box to supply the whole car. I ended up using only 3 relays for the engine side of the harness but then split them out using fuses for each separe component to narrow down a problem. The fuse/relay board i used is MTA modular fuse and relay systems, handy as you can buy the modules to suit and can use busbars to split a power source easily ove multiple fuses. I added separated dt connectors for easy removal of the box. Ive attached a few photos. I still need to put in fuses and relays and drill out holes and add switches. Im pretty happy with the result however on my next build i am going to use a PDM.

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